Field-effect transistors (FET) has been considered to be an ideal technology for nonvolatile memory because of its random access, high speed, low power, high density and simplicity. For a nonvolatile semiconductor storage device in which each memory cell is composed of an FET provided with a floating gate covered by an insulating film and used as a charge storing layer, data is stored by controlling the amount of electrons stored in the floating gate thereby changing the threshold voltage of the transistor. When programming or erasing data into or from the memory cell, electrons are either injected or ejected from the floating gate via the insulating film.
Electron injection/ejection is possible by using the (Fowler-Nordheim (F-N)) tunnel phenomenon and the hot electron phenomenon. Electrons are injected in the insulating film around the floating gate with the application of a high electrical field. Consequently, when the number of program/erase cycles is increased, the insulating film receives an electron injection stress repetitively, thereby degrading the insulating film.
This results in the degradation of the various properties of the memory cell. Especially, when a low electrical field is applied to the insulating film that has been degraded due to such repetitive program/erase operations, the leakage current (low electric field leakage current or stress-induced leakage current) is increased. Consequently, the electron retention characteristics or the disturb characteristics of the memory cell are degraded. This degradation has now been questioned as a factor to limit the program/erase cycles for such a nonvolatile semiconductor memory device.
It is known that many charged trapping centers are formed in the gate insulating film depending on the program/erase operation method if the program/erase operation is repeated. When electrons pass through the insulating film during program/erase operation, some of the electrons are trapped by those charged trapping centers. Those trapped electrons leak more easily out of the insulating film than the electrons stored in the floating gate. The threshold voltage of the memory cell is thus varied sharply and quickly after a program/erase operation, thereby affecting the data retention characteristics.